In recent years, an image reading device including a sheet transport device capable of transporting a plurality of sheets like documents to an image reading section of an image reading device and causing the image reading section to sequentially read the sheets has been used.
In transporting a sheet, the sheet transport device may transport a skewed sheet (skews the sheet), without transporting a sheet straight. For example, when the image reading section reads an image on the skewed sheet, the image is read in a displaced state, or a portion of the image on the sheet becomes unreadable. This would cause degradation in quality of reading.
In view of this, the sheet transport device is provided with a resist section for straightening a skew of the sheet at the position where a sheet is delivered to the image reading section. By using this resist section, a timing of reading the sheet can be adjusted.
Note that, Japanese Laid-Open Patent Application No. 108863/1983 (Tokukaisho 58-108863; published on Jun. 29, 1983) discloses a two-sided recording system having such an arrangement in which two printing sections are provided in a transport path for documents, and each of the printing sections is provided with a resist section only on the front end.
However, in the conventional arrangement, there is the possibility that a skew caused would become so large to be straightened sufficiently.
The skew of the sheet transported occurs when the sheet is picked up from a tray or during a period of time in which a sheet is being transported in the transport path.
For example, a long transport path for sheets causes a skew due to a size error of a transport member (transport roller) provided in the transport path and instability (variations) of contact load of a transport guide.
Further, a pickup roller of such a shape that the roller only comes into contact with the central part of the sheet is used so that sheets of any sizes can be picked up from a tray. With this arrangement, picking-up operation becomes unstable depending on a state where the sheet is placed in the tray and variations in contact load (state) between sheets, resulting in the occurrence of a skew.
While a greater number of sheets can be placed once in a tray, reduction in size of a main unit of a device is required. In view of the circumstances, such a sheet transport device is used that a transport path through which a sheet passes from a tray to an image reading section is curved to attain the reduction in size of the main unit of the device. In such an arrangement in which the transport path is curved, since a sheet is transported in a bowed state, a skew of the sheet is more likely to occur.
For this reason, when a sheet reaches the position of the resist section placed in the front of the image reading section, a skew has already become large, which causes the possibility that the skew could not be sufficiently straightened at the resist section.
The present invention is attained in view of the above-mentioned problems, and provides a sheet transport device which can reliably straighten skewing of a sheet to be transported, an image reading device, and an image forming device.